Properties of a Bacillus subtilis mutant able to sporulate continually during growth in synthetic medium

Several mutants of Bacillus subtilis were isolated which sporulate continually during exponential growth in glucose medium. The spdA1 mutation, responsible for the continual sporulation of one of the mutants, mapped near thr. When an exponentially growing culture of a strain containing spdA1 was maintained at essentially constant turbidity, 5% of the viable cells contained heat-resistant spores. The continual sporulation depended on the stringent response since it was absent in spdA relA double mutants. Genetic and biochemical analysis indicated that the continual sporulation of spdA1 strains was associated with a lower specific activity of pyruvate carboxylase, which limited the rate of oxaloacetate synthesis from glucose via pyruvate and thereby the supply of compounds depending on the citrate cycle, especially aspartate. Therefore, the mild stringent response caused by the spdA1 mutation seems to result from a partial deficiency of aspartyl-tRNA which may exert its sporulation-initiating effect during a limited time interval in each growth cycle. A mutant blocked in fumarase activity (citG) behaved similarly. It grew only slowly in glucose medium because much of the limiting oxaloacetate was wasted for the excretion of fumarate. The mutant produced little aspartate and sporulated at a high frequency in glucose medium, even in the presence of glutamate; the sporulation was again prevented by aspartate or malate or by introduction of the relA marker into the strain.